1. Technical Field
The present invention relates to a hydraulic oil composition used in hydraulic devices etc. in the fields of architecture etc. and in particular to a hydraulic oil composition excellent in oxidative stability and lubricating properties and superior in biodegradability.
2. Description of the Prior Art
The hydraulic oil used in hydraulic devices such as constructing machines may be leaked into soil or rivers so that biodegradable hydraulic oil is often used for the environment. As the base oil in the biodegradable hydraulic oil, synthetic esters and vegetable oil are generally used, but the synthetic esters have the problem of higher prices and the synthetic esters excellent in biodegradability have the disadvantage of being inferior in oxidative stability. On the other hand, the vegetable oil is naturally occurring oil and is thus excellent in biodegradability and superior in respect of lower prices, but it is poor in oxidative stability, and because the conventional hydraulic oil using said vegetable oil as the base oil has the problems of its easily increasing viscosity during its use, easy formation of sludge, etc., it is difficult to use the conventional hydraulic oil under severe environments such as high temperatures, high loading and the like.